On an MRI scan, normal pressure hydrocephalus (NPH) presents a distinctive pattern: enlarged ventricles—the fluid-filled spaces in the brain—accompanied by a narrowed space over the top of the brain where cerebrospinal fluid normally circulates freely. This is the hallmark appearance that radiologists look for, and it often distinguishes NPH from other forms of dementia. A patient with NPH might have ventricles that look like inflated balloons, particularly in the frontal horns (the forward extensions of the lateral ventricles), while simultaneously showing paradoxically tight sulci (grooves) on the brain’s surface. The contrast between enlarged internal spaces and compressed external spaces is the critical visual clue. The technical reason this happens is that cerebrospinal fluid becomes trapped in the ventricles due to impaired reabsorption, creating pressure that pushes the ventricle walls outward.
Unlike normal aging, where the brain shrinks and ventricles enlarge proportionally, NPH shows a disproportionate enlargement—the ventricles grow larger than the degree of brain atrophy would explain. This mismatch is what makes NPH recognizable to experienced radiologists. An important caveat: MRI alone cannot diagnose NPH. A scan might show the characteristic appearance, but the diagnosis requires the clinical triad of gait disturbance, cognitive decline, and urinary incontinence, along with imaging confirmation. Many people have the imaging findings without the disease, which is why understanding what the scan actually shows—and what it means—matters for both patients and care partners.
Table of Contents
- THE HALLMARK VENTRICLE ENLARGEMENT AND TEMPORAL HORN DILATION
- THE DESH PATTERN AND BLACK TENT SIGN—THE MOST SPECIFIC MRI INDICATORS
- HOW NPH IMAGING DIFFERS FROM OTHER DEMENTIAS AND NORMAL AGING
- THE EVANS INDEX AND QUANTIFYING VENTRICULAR SIZE
- AQUEDUCTAL FLOW VOIDS AND CSF DYNAMICS ON MRI
- CORPUS CALLOSUM CHANGES AND BRAIN TISSUE DISPLACEMENT
- THE CRITICAL LIMITATION—WHY MRI ALONE CANNOT CONFIRM NPH DIAGNOSIS
THE HALLMARK VENTRICLE ENLARGEMENT AND TEMPORAL HORN DILATION
NPH’s most striking MRI feature is ventricomegaly—enlargement of the ventricles—that appears disproportionate to any shrinkage of the brain tissue itself. The frontal horns, which are the anterior tips of the lateral ventricles, often show characteristic ballooning. Simultaneously, the temporal horns (the portions of the ventricles that extend into the temporal lobes near the ears) appear notably widened. This temporal horn enlargement is particularly important because it helps distinguish NPH from other dementias. In Alzheimer’s disease, for example, the hippocampus typically shrinks, which would proportionally narrow the temporal horns; in NPH, the temporal horns stay dilated despite normal hippocampal size, creating a distinctive visual signature. Periventricular hyperintensities—bright white areas on T2-weighted MRI sequences that outline the ventricles—reflect cerebrospinal fluid leaking across the ventricular wall into the brain tissue itself.
This transependymal CSF flow is evidence that the pressure inside the ventricles is pushing fluid outward. Think of it as a warning light: the brain tissue around the ventricles is becoming waterlogged. These hyperintensities are not unique to NPH and appear in other conditions, but their presence combined with the ventricle pattern is significant. One limitation to remember: on any given MRI, these features exist on a spectrum. Some people show pronounced enlargement; others show mild changes. A radiologist’s interpretation depends partly on the quality of the imaging and their experience with NPH. This is why quantitative measures—numerical measurements rather than just visual assessment—have become increasingly important in diagnosis.
THE DESH PATTERN AND BLACK TENT SIGN—THE MOST SPECIFIC MRI INDICATORS
The DESH pattern (Disproportionately Enlarged Subarachnoid Space Hydrocephalus) is a specific MRI finding that has become a cornerstone of NPH diagnosis. DESH refers to the paradoxical combination of enlarged ventricles with simultaneously narrowed sulci over the brain’s convexities—the top and sides of the brain. On the MRI, the brain’s natural grooves look compressed or nearly absent while the internal ventricles bulge outward. This pattern suggests that intracranial pressure is pushing inward on the brain’s surface. When a radiologist sees DESH, NPH becomes a leading diagnostic consideration. The black tent sign is even more specific. This finding appears on 3D T2 SPACE imaging (a specialized MRI sequence) at the level of the anterior commissure and shows as a characteristic hypointense (dark) signal.
The “tent” refers to the shape of the corpus callosum when it’s elevated and compressed by enlarged ventricles—it creates a peaked appearance on cross-sectional images. The black tent sign has high sensitivity and specificity for NPH, meaning when it’s present, NPH is more likely to be the correct diagnosis. However, the sign requires specific imaging sequences and expertise to identify; not all MRI facilities routinely look for it. One important warning: the presence of DESH or a black tent sign does not automatically confirm NPH. These findings must be interpreted alongside clinical symptoms. A patient without gait problems, cognitive decline, or incontinence may have these imaging findings but not have NPH—a phenomenon sometimes called “probable iNPH without symptoms” or subclinical disease. This distinction matters because not everyone with the imaging appearance will benefit from the treatment (shunt placement), which is why clinical assessment is essential.
HOW NPH IMAGING DIFFERS FROM OTHER DEMENTIAS AND NORMAL AGING
The key difference between NPH and Alzheimer’s disease on MRI is the pattern of changes. Alzheimer’s typically shows hippocampal atrophy (shrinkage) with proportionate ventricular enlargement—larger ventricles because the brain is smaller overall. In NPH, the ventricles are large but the hippocampus and overall brain size are relatively preserved, creating the mismatch that defines the condition. A radiologist comparing these two patterns would see Alzheimer’s as a uniform shrinkage, while NPH appears as selective ventricle swelling without proportional brain loss. Normal aging presents yet another pattern: modest ventricular enlargement and widened sulci, reflecting the gradual brain atrophy that occurs in all of us as we age. The key distinction is that in normal aging, the sulci remain prominent and visible; in NPH, the sulci are paradoxically narrowed despite the ventricle enlargement.
If you imagine the brain as a sponge, normal aging is like the sponge shrinking overall, while NPH is like fluid being pumped into the internal cavities while the external surface is compressed. This distinction is crucial because it prevents misdiagnosis of NPH as simple age-related cognitive decline. Distinguishing between these patterns requires skill and sometimes serial imaging (comparing scans over time). A patient with NPH who receives a lumbar puncture often shows clinical improvement afterward, which helps confirm the diagnosis. In contrast, patients with Alzheimer’s or normal aging do not improve after lumbar puncture. This is why the clinical test—temporary removal of cerebrospinal fluid—serves as confirmation when imaging alone is ambiguous.
THE EVANS INDEX AND QUANTIFYING VENTRICULAR SIZE
The Evans index is a numerical measurement that quantifies ventricular enlargement. It is calculated as the ratio of the maximum width of the frontal horns of the lateral ventricles to the maximum width of the intracranial diameter (measured at the same level). The formula is simple: frontal horn width divided by intracranial width. An Evans index of 0.30 or greater is considered abnormal and is the gold standard quantitative criterion for ventricomegaly. An index of 0.40 or higher indicates more severe enlargement. Why use a number instead of just looking at the image? Because the Evans index provides objectivity. Two radiologists might have different impressions looking at a scan visually, but both can measure the same ventricle width and skull width. This standardization is especially important when tracking progression over time or comparing across institutions.
For example, a patient might have an Evans index of 0.32 on an initial scan, 0.35 on a follow-up six months later, and 0.38 after a year—indicating gradual progression. Without numerical measurement, subtle changes might be missed. However, the Evans index has limitations. It measures only the frontal horns and does not account for overall brain size variation among individuals. A person with a naturally large head might have a higher Evans index without having NPH. Similarly, someone with mild brain atrophy might have a lower index even though NPH is present. This is why the Evans index is most useful when combined with clinical evaluation and other imaging findings like the DESH pattern or black tent sign. No single measurement tells the complete story.
AQUEDUCTAL FLOW VOIDS AND CSF DYNAMICS ON MRI
On MRI, cerebrospinal fluid normally appears as a dark signal (hypointense) when the fluid is moving. The aqueduct of Sylvius is the narrow passage connecting the third and fourth ventricles, and in NPH, characteristic flow voids—areas of altered signal representing turbulent or increased CSF flow—appear at the aqueductal level. Similarly, the fourth ventricle (the small chamber at the base of the brain) may show increased flow voids. These findings represent the brain’s attempt to compensate for impaired CSF absorption by increasing flow velocity through the aqueduct. The presence of aqueductal and fourth ventricular flow voids is considered supportive of NPH diagnosis, but it is not specific to the condition. Patients with other causes of ventricomegaly can also show these findings.
Additionally, advanced MRI techniques like phase-contrast cine MRI can quantify CSF flow rates through the aqueduct, and some research suggests that elevated aqueductal flow correlates with NPH. However, quantified flow measurements are not yet part of standard diagnostic criteria and are mainly used in research settings. One practical consideration: MRI sequences that show flow voids require specific timing and imaging parameters. Some MRI centers perform routine imaging that may not optimally visualize these flow characteristics. If CSF dynamics are clinically important for diagnosis, the referring physician should request sequences specifically designed to evaluate aqueductal flow. This is an area where communication between the clinical team and radiology department can significantly affect diagnostic accuracy.
CORPUS CALLOSUM CHANGES AND BRAIN TISSUE DISPLACEMENT
The corpus callosum is the largest white matter tract in the brain, connecting the left and right hemispheres. In NPH, the corpus callosum often appears thinned and elevated—pushed upward by the expanding ventricles beneath it. On a midline MRI image (a slice down the center of the brain), the corpus callosum might appear peaked or tent-shaped, reflecting the pressure from below. This finding is visually striking and helps radiologists immediately recognize the condition.
Combined with the narrowed sulci, the elevated corpus callosum creates a distinctive overall appearance that experienced neuroradiologists recognize quickly. The thinning of the corpus callosum in NPH reflects both mechanical compression and, potentially, some degree of tissue change from chronic CSF pressure exposure. Research using diffusion tensor imaging (a specialized MRI technique) has shown white matter changes in NPH patients that partially recover after shunt placement, suggesting that some of the changes are reversible. This is clinically relevant because it implies that early diagnosis and treatment might prevent irreversible damage, whereas long-standing untreated NPH could lead to permanent white matter changes.
THE CRITICAL LIMITATION—WHY MRI ALONE CANNOT CONFIRM NPH DIAGNOSIS
Despite its diagnostic power, MRI cannot definitively confirm NPH on its own. The imaging may look textbook-perfect—enlarged ventricles, DESH pattern, black tent sign, elevated corpus callosum—yet the patient might not have NPH because they lack the clinical symptoms. The American Academy of Neurology guidelines specify that probable iNPH (idiopathic normal pressure hydrocephalus) requires at least one symptom from the triad (gait disturbance, cognitive decline, or urinary incontinence), compatible imaging findings, and objective gait improvement following high-volume lumbar puncture. The lumbar puncture is the functional test: removing 40 to 50 milliliters of cerebrospinal fluid temporarily alleviates symptoms if NPH is present, and improvement confirms the diagnosis.
This is where the clinical reality becomes more complex than the imaging alone suggests. A 75-year-old with mild memory problems might have an MRI showing clear ventricomegaly and DESH pattern. But if their gait is normal and there are no urinary symptoms, they may not have NPH—they may have Alzheimer’s disease, normal aging, or another condition. Conversely, another patient with the textbook NPH symptoms but only borderline imaging findings could still have the disease. The radiologist’s job is to provide the imaging information; the clinician’s job is to synthesize that with the clinical picture and, when indicated, recommend the lumbar puncture test that can confirm or refute the diagnosis.
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